Dispersion atmospheric tracers

Amts et al. (1982) have measured the dispersion of alpha-pinene from a loblolly pine forest with the help of SF6, which was released inside the forest and served as a tracer for air motions. Both SF6 and alpha-pinene were measured downwind at a number of receptor sites. An atmospheric plume dispersion model was used to describe the observations and to infer emission rates. Losses of alpha-pinene due to reactions with ozone and OH radicals were incorporated into the model. Emission rates derived ranged from 11 to 19 pg/m2 min. No direct comparison was made with the rate of alpha-pinene emissions from three branches by the bag enclosure technique. Instead, available data from Rasmussen (1972) and Zimmerman (1979b) were used to estimate the rate expected for this particular forest site at the temperatures encountered during the measurements. The values obtained were 14-35 p.g/m2 min and 47 p.g/m2 min, respectively. Although the alpha-pinene fluxes inferred from the tracer release approach are lower, the differences are not great enough in view of the large variability of actual emission rates to raise doubts about the applicability of the bag enclosure technique. 

Validating the final experimental protocol was accomplished by running a model study in which Nd was released into the atmosphere from a 100-MW coal utility boiler. Samples were collected at 13 locations, all of which were 20 km from the source. Experimental results were compared with predictions determined by the rate at which the tracer was released and the known dispersion of the emissions. 

Tracer studies are extremely important in furthering our knowledge of atmospheric dispersion. These studies consist of release of a known quan-... 

The aim of dispersion models is the prediction of atmospheric dilution of pollutants in order to prevent or avoid nuisance. Established dispersion models, designed for the large scale of industrial air pollution have to be modified to the small scale of agricultural pollutions. An experimental setup is described to measure atmospheric dilution of tracer gas under agricultural conditions. The experimental results deliver the data base to identify the parameters of the models, For undisturbed airflow modified Gaussian models are applicable. For the consideration of obstacles more sophisticated models are necessary,... 

The published guideline VDI 3881 /2—4/ describes, how to measure odour emissions for application in dispersion models. Results obtained by this method have to be completed with physical data like flow rates etc. As olfactometric odour threshold determination is rather expensive, it is supplemented with tracer gas emissions, easy to quantify. In the mobile tracer gas emission source, fig, 2, up to 50 kg propane per hour are diluted with up to 1000 m2 3 air per hour. This blend is blown into the open atmosphere. The dilution device, including the fan, can be seperated from the trailer and mounted at any place, e.g. 

Equation (2.2) can be considered as the fundamental governing equation for the concentration of an inert constituent in a turbulent flow. Because the flow in the atmosphere is turbulent, the velocity vector u is a random function of location and time. Consequently, the concentration c is also a random fimction of location and time. Thus, the dispersion of a pollutant (or tracer) in the atmosphere essentiaUy involves the propagation of the species molecules through a random medium. Even if the strength and spatial distribution of the source 5 are assumed to be known precisely, the concentration of tracer resulting from that source is a random quantity. The instantaneous, random concentration, c(x, y, z, t), of an inert tracer in a turbulent fluid with random velocity field u( c, y, z, t) resulting from a source distribution S x, y, z, t) is described by Eq. (2.2). 

In this study we have employed the simultaneous collection of atmospheric particles and gases followed by multielement analysis as an approach for the determination of source-receptor relationships. A number of particulate tracer elements have previously been linked to sources (e.g., V to identify oil-fired power plant emissions, Na for marine aerosols, and Pb for motor vehicle contribution). Receptor methods commonly used to assess the interregional impact of such emissions include chemical mass balances (CMBs) and factor analysis (FA), the latter often including wind trajectories. With CMBs, source-strengths are determined (1) from the relative concentrations of marker elements measured at emission sources. When enough sample analyses are available, correlation calculations from FA and knowledge of source-emission compositions may identify groups of species from a common source type and identify potential marker elements. The source composition patterns are not necessary as the elemental concentrations in each sample are normalized to the mean value of the element. Recently a hybrid receptor model was proposed by Lewis and Stevens (2) in which the dispersion, deposition, and conversion characteristics of sulfur species in power-plant emissions... 

Because of greatly contrasting low tritium levels before thermonuclear tests and because of the distinct peak tritium levels that occurred in the atmosphere during 1962-1965, tritium has been used as an environmental tracer in the studies on surface water budgets, groundwater age and flow velocities, groundwater recharge, and dispersion and diffusion in aquifers. A 1989 study detailed the distribution of tritium... 

Transport and dispersion was evaluated without any form of tuning by comparing a simulation of the ETEX-1 release to the official measurements of surface concentration. To facilitate comparisons with models evaluated during ATMES 11 (Atmospheric Transport Model Evaluation Study) an identical statistical methodology was employed (Mosca et al. 1998). Background values were subtracted so that only the pure tracer concentration was used. Measurements of zero concentration (concentrations below the background level) were included in time series to the extent that they lay between two non-zero measurements or within two before or two after a non-zero measurement. Hereby, spurious correlations between predicted and measured zero-values far away from the plume track are reduced. 

Analysis and tabulations of data to be used in Gaussian plume formulas are also available. The report for the St. Louis Dispersion Study (13) gives further insight into tracer-spreading over urban areas in contradistinction to open areas where many measurements have been made. Detailed working charts and numerous examples in Turners workbook (14) aid practical estimation of atmospheric dispersions under the conditions outlined above. 

Following injection of radioactive debris into the atmosphere, the subsequent dispersion and eventual deposition of the material onto the earth s surface are determined by mass air circulation patterns in the atmosphere. These patterns have been established from meteorological studies and from measurements of the behaviour of fallout radionuclides. The injection of radionuclides into the atmosphere by atmospheric weapons testing has provided a unique tracer experiment, which has helped to provide understanding of the atmospheric processes. 

Rappolt, T. (2001) Field test report measurements of atmospheric dispersion in the Los Angeles urban environment in summer 2001, Report number 1322, prepared for STI, Bel Air, MD, by Tracer Environ. Sci. and Tech., San Marcos, CA 92071. 33 pp. plus data CD. 

Much can be learnt from the direct observation of atmospheric dispersion phenomena. Many observations are of the dispersion of a plume of visible tracer (e.g. smoke or water vapour) from a chimney or stack , such that it is quite the convention to describe dispersion in this context. Accordingly, stack-plume observation is used here to help identify the main processes of atmospheric dispersion pertinent to the distribution and delivery of CWAs. In the following, these processes are described under four headings ... 

The stability of the atmosphere is clearly a major influence on the dispersion of a smoke-plume. If the plume comprised a CWA rather than a harmless tracer, then the atmospheric stability would have an important effect on the extent of the downwind hazard area and on the concentrations of agent experienced within that hazard area. Atmospheric stability can be determined by direct observation - as in Figures 1-5 -or by estimation of the Monin-Obukhov length-scale, L (m), which is a height proportional to the height above a surface at which thermal effects first dominate shear (momentum) effects (Pasquill, 1962), as defined in Equation (1) ... 

As indicated, the flux may be expressed either in units of molecules/m2 s or in units of kg/m2 s. Here, p and n are the density and number density of air, respectively, and K is called the eddy diffusion coefficient. This quantity must be treated as a tensor because atmospheric diffusion is highly anisotropic due to gravitational constraints on the vertical motion and large-scale variations in the turbulence field. Eddy diffusivity is a property of the flowing medium and not specific to the tracer. Contrary to molecular diffusion, the gradient is applied to the mixing ratio and not to number density, and the eddy diffusion coefficient is independent of the type of trace substance considered. In fact, aerosol particles and trace gases are expected to disperse with similar velocities. 

Bauer, E. (1974). Dispersion of tracers in the atmosphere and ocean survey and comparison of experimental data. J. Geophys. Res. 79, 789-795. 

CFCs, and Kr were studied in a sandy, unconfined aquifer on the Delmarva Peninsula in the eastern USA by Ekwurzel et al. (1994). H and H+ He depth-profiles show peak-shaped curves that correspond to the time series of H concentration precipitation, smoothed by dispersion (Fig. 18a). The peak occuring at a depth of about 8m below the water table therefore most likely reflects the H peak in precipitation that occurred in 1963 (Fig. 6). The H- He ages show a linear increase with depth, reaching a maximum of about 32 years. The H- He ages are also supported by CFC-11, CFC-12, and Kr tracer data (Fig. 18b). The latter tracers are used here as dyes and their concentrations are converted into residence times by using the known history of the atmospheric concentrations and their solubility in water. From the vertical H- He age profile at well nest 4 at the Delmarva site, the vertical flow velocity can be... 

Conventional Tracers. A survey of the history of environmental science will show that tracers are important tools for the environmental scientist. They have been used to measure flow rates and dispersion coefficients, to follow the movement of materials through the atmosphere, hydrosphere, and biosphere, and to characterize pollutant sources. Three principal types of tracer have been used in environmental science chemical tracers, such as NaCl, KHSO4, and K2CO3 radioactive tracers, such as H, and Br and fluorescent dyes, such as fluorescein, Rhodamine B, and Rhodamine WT. 

It turns out that turbulent diffusion can be described with Fick s laws of diffusion that were introduced in the previous section, except that the molecular diffusion coefficient is to be replaced by an eddy or turbulent diffusivity E. In contrast to molecular diffusivities, eddy dififusivities are dependent only on the phase motion and are thus identical for the transport of different chemicals and even for the transport of heat. What part of the movement of a turbulent fluid is considered to contribute to mean advective motion and what is random fluctuation (and therefore interpreted as turbulent diffusion) depends on the spatial and temporal scale of the system under investigation. This implies that eddy diffusion coefficients are scale dependent, increasing with system size. Eddy diffusivities in the ocean and atmosphere are typically anisotropic, having much large values in the horizontal than in the vertical dimension. One reason is that the horizontal extension of these spheres is much larger than their vertical extension, which is limited to approximately 10 km. The density stratification of large water bodies further limits turbulence in the vertical dimension, as does a temperature inversion in the atmosphere. Eddy diffusivities in water bodies and the atmosphere can be empirically determined with the help of tracer compounds. These are naturally occurring or deliberately released compounds with well-estabhshed sources and sinks. Their concentrations are easily measured so that their dispersion can be observed readily. 

Fares et al. (1980) were the first to apply a general Gaussian plume model (3.5) to pheromone dispersion and the first to emphasize the importance of atmospheric stability on pheromone communication. They used dispersion coefficients derived from tracer experiments conducted in a pine forest. They hypothesized that the diurnal pattern of bark beetle responses to pheromone may relate to diurnal changes in stability. (For further discussion of the selective forces influencing diurnal activity patterns see Card6 and Baker, Chapter 12.)... 

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